1. Field of the Invention
The present invention relates to a process for producing uncrosslinked polypropylene-based resin foam sheets which have fine foam cells and good appearance. More specifically, the present invention relates to a process for producing uncrosslinked polypropylene-based resin foam sheets, characterized in that the productivity in the production of high-expansion thin sheets is remarkably increased.
2. Description of the Related Art
A technique, in which extrusion foaming is carried out by interposing a gear pump between an extruder and a die, has been hitherto applied to a process for producing polystyrene and is described in, for example, "Plastic Technology 28 [2] 87 (1980)".
The effects of using a gear pump in extrusion forming are as follows according to "Synthetic Resins 40 [9] 42 (1994)".
(1) Effects brought about by the suppression of pressure rise in the extruder PA0 (2) Influences of combination with die are eliminated. PA0 (3) Stabilization in production PA0 1. A process for producing uncrosslinked polypropylene-based resin foam sheets which is characterized by using an extrusion foaming apparatus having a gear pump which is interposed between the extruder and the die and which is linked by means of adapters, wherein the pressure (Pg) at the inlet of the gear pump is within the range specified by the following formula (Formula 1) and the extruder is controlled so that the value of Pg is constant. PA0 2. A process for producing uncrosslinked polypropylene-based resin foam sheets according to above 1, wherein the die is characterized by 5.ltoreq.L/t.ltoreq.12 where L (mm) stands for the length of lip land and t (mm) stands for the lip gap. PA0 3. A process for producing uncrosslinked polypropylene-based resin foam sheets according to above 1 or 2, wherein the extruding apparatus is characterized by 1/4.ltoreq.A/B.ltoreq.1and C/A.ltoreq.10 where the B (mm) stands for the barrel diameter, C (mm) stands for the length of the adapter, and A (mm) stands for the adapter diameter. PA0 4. The uncrosslinked polypropylene-based resin foam sheet produced by a process described in any one of above 1to 3.
reduction of energy consumption PA1 prevention of rise in resin temperature PA1 improvement of transfer efficiency (productivity) PA1 reduction of retention time in the extruder PA1 reduction of L/D of the extruder PA1 The setting of temperature, pressure, and revolution number of the extruder becomes easier. PA1 Irrespective of the shapes of die, the conditions set to the extruder do not need to be changed if the resin to be used and the production amount are unchanged. PA1 The difference of product qualities due to difference of the abilities of operators is eliminated. PA1 Unmanned operation and automated operation become possible. PA1 The influence of pressure change in the extruder is eliminated. PA1 Even moistened raw materials can be used. PA1 Temperature distribution becomes nearly constant. PA1 Time required for the start up of production can be remarkably reduced.
However, all of the above-described effects are not necessarily obtained by using a gear pump because processing conditions suited for foaming vary depending on resins and because wavy or corrugated defects peculiar to foaming tend to occur at ends of sheets in the case of extrusion foaming.
In particular, in the case of an uncrosslinked polypropylene-based resin which is crystalline and whose viscosity sharply drops at a temperature higher than the melting point, it is impossible to produce high-expansion foam sheets having uniform fine foam cells by conventional extrusion foaming. Further, an attempt to increase delivery rate in order to increase productivity or to produce high-expansion foam sheets is associated with problems such as rise in resin pressure and temperature.
The loss due to wavy or corrugated defects at ends of sheets occurs when transverse expansion due to foaming is regulated by the width of die. This phenomenon is peculiar to foamed sheets, particularly a high-expansion foamed sheet by an expansion ratio of 2 or greater. In addition, the loss due to wavy or corrugated defects at ends of sheets occurs frequently when the resin temperature is high and the foaming gas tends to be leaked. Therefore, a need exists for means to reduce the above loss in the production of uncrosslinked polypropylene-based resin foam sheets.